Nonlinear Sound Waves in Ultracold Fermi Fluids

Studies of sound propagation have been fruitful in revealing various thermodynamic and transport properties of its medium, such as compressibility and diffusivity. Much of the activity has focused on the linear-response regime. Here we report the excitation and observation of nonlinear sound waves in degenerate Fermi gases. We apply an oscillating spatially homogeneous force to a uniform spin-balanced Fermi gas and measure the displacement of the center-of-mass (CoM) versus the driving frequency. We observe different signatures of the emergence of nonlinear acoustic response in both the weakly and strongly interacting Fermi gases.